The goal of this research is to investigate the effects of stress on childhood brain development. While low levels of stress during development can promote healthy coping later in life, chronic stress sets up vulnerabilities for psychiatric disorders and impairs cognition. Although chronic stress cannot be experimentally manipulated in humans, it is possible to investigate the effects of naturally occurring variations in stress exposure. Childhood socioeconomic status (SES), an index composed of parental education, occupation, and income, is linked to a gradient in exposure and reactivity to stress. Lower SES is associated with more stressful life events and greater dysregulation of stress physiology. Further, low SES environments are associated with fewer factors that buffer the effects of stress. Recent animal research has provided evidence that stress might accelerate important maturational processes. This is problematic in light of the finding in humans that slower brain development is associated with superior cognitive skills. I propose to test the novel hypothesis that stress accelerates brain development in young children. Further, I will explore the question of whether early brain maturation is an adaptive response to a high-stress environment. To test these hypotheses, I will recruit six-year-old children from low SES backgrounds who have experience many stressful life events (high-stress group, N=40), and children from high SES backgrounds who have experienced few stressful life events (low-stress group, N=40). I will 1) characterize differences in perceived stress, physiological stress markers, and cognitive performance between high- and low-stress groups; 2) test whether the high-stress group exhibits greater structural brain maturity as indexed by greater cortical thickness and more directional diffusivity in white matter; 3) explore whether structural maturation differences are reflected in functional networks; and 4) investigate whether early maturation in the high-stress group is associated with inferior or superior cognitive abilities and mental health. In high-stress environments, accelerated maturation may be adaptive because it protects networks supporting cognitive skills in a partially developed state, rendering them less vulnerable to environmental insult. In sum, this research will be the first to test the innovative hypothesis that stress accelerates brain development in young children, and to explore whether accelerated development is an adaptive response to stress. Results of this study will help fill a major gap in knowledge that has evolved from the underrepresentation of children from low SES backgrounds in cognitive neuroscience research. Our findings will elucidate the neurodevelopmental mechanisms by which childhood stress impacts cognition and increases risk for psychiatric illness. Knowledge gained from this work will provide insight into whether interventions should aim to accelerate or decelerate brain maturation in children exposed to stress.